Computer storage systems are usually provided on discs of some sort for example magnetic or optical discs. Permanent storage is normally provided on hard discs. These can range from the hard disc provided within a personal computer to multiple disc systems used on large networks of the type which might be used by for example banks.
In the case of hard disc systems, such as RAID (Redundant Arrays of Inexpensive Drives) devices, memory is divided into what are known as disc volumes. A disc volume may comprise a single disc, a subset of a single disc, or a number of discs, depending on the application in question. Each disc volume is treated by the computer system as a number of blocks of storage which are usually equally sized tracks (e.g. 32K bytes) wherein each track is a discrete addressable portion of the disc volume.
It is standard practice to make backups of data from disc volumes at regular intervals. These enable data to be recovered in the event of a system failure. Also, they allow some off-line analysis to be performed on data.
One of the drawbacks of backup copies is that they take a very long period of time to compile, this being dependent on the size of the disc volumes being backed up. Also during the time within which the backup is being compiled, some of the data may change. The backup will therefore be made with no knowledge of this.
It has been appreciated that in addition to the normal backup copy it would be useful to have a copy of a disc volume which is a true copy at a discrete point in time. This is known as a point in time copy.
At present, it is only possible to take a single point in time copy. The taking of a further point in time copy erases the earlier point in time copy. In some applications, multiple point in time copies would be useful so that data can be taken off-line and subsequently analysed, e.g. to look for trends in particular pieces of data. At present, it is necessary to download a copy point in time copy before a new one can be made and the download time may be longer than the interval required between point in time copies.